High frequency attenuator



P 1967 E. E CANDILIS 3,341,790

HIGH FREQUENCY ATTENUATOR Filed Dec. 9, 1965 INVENTOR EMMANUEL E. CANDILIS @cgwm AGENT United States Patent 3,341,790 HIGH FREQUENCY ATTENUATOR Emmanuel E. Candilis, Palo Alto, Calif., assignor to Hewlett-Packard Company, Palo Alto, Calif., a corporation of California Filed Dec. 9, 1963, Ser. No. 329,041 6 Claims. (Cl. 333-81) This invention relates to high frequency attenuators which provide continuously variable attenuation with low insertion loss.

High frequency attenuators are widely used as precision calibrators and in direct comparison measurements. It is generally desirable in these applications that an attenuator provide continuously variable attenuation from Zero decibel for a broad band of frequencies. Further, it is desirable that an attenuator be small in size and of simple, low cost construction. Additionally, it is important that the input and output impedances of the attenuator remain constant with variation in attenuation for the entire band of frequencies to provide a good match. Certain deposited film attenuators in the prior art include a resistive T network wherein three variable resistors are required to provide substantially constant input and output impedance with variation in attenuation.

Accordingly, it is an object of this invention to provide a high frequency attenuator which affords continuously variable attenuation with low insertion loss for a broad band of frequencies.

It is another object of this invention to provide a low cost attenuator which is small in size and simple in construction.

A further object of this invention is to provide a deposited film attenuator including a resistive bridged T network wherein only two variable resistors are required to provide constant input and output impedance with variation in attenuation. i

In accordance with the illustrated embodiment of this invention there is provided a surface having an attenuation network deposited thereon in a configuration calculated to provide a constant input and output impedance with variation in attenuation. This network is provided with a signal input and a signal output terminal. Spring loaded electrical contacts are positioned in a contiguous relationship with the attenuation network. The electrical contacts are disposed for cooperation with the attenuation network to provide continuously variable attenuation with low insertion loss in response to relative movement therebetween.

Other and incidental objects of this invention will be apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIGURE 1 is an exploded view of an attenuator according to thi invention; and

FIGURE 2 is a schematic diagram of the attenuator of FIGURE 1.

Referring to FIGURES l and 2, there is shown a bridged tee attenuation network 10 which is deposited with uniform thickness on the surface of -a disc 12 of dielectric material, for example, glass. The bridge resistor 14 is deposited with varying width according to the relationship 1 (97.7l9 l() "+p/t) where W is the width of the deposited resistor 14 in meters, r is the radius of its outermost edge in meters, t is its thickness in meters, 6 is its clockwise rotation in radians, and p is its resistivity in ohm-meters. This relationship for W has been derived for an input and output impedance equal to fifty ohms and an attenuation range of Zero to forty decibels for a clockwise rotation from zero to two hundred seventy degrees. A conductive strip 16 is deposited with uniform width adjacent the outermost edge of resistor 14. Resistors 18 and 20 have the same value and are deposited with uniform width on an are having a lesser radius than the innermost point of the resistor 14. A conductive element 22 connects the widest end of the resistor 14 and one end of the resistor 18. It also forms the input of the attenuation network 10 and is connected to the signal input terminal 24. Similarly, the conductive element 26 connects the narrowest end of the resistor 14 to one end of the conductive strip 16 and one end of the resistor 20. It also forms the output of the attenuation network 10 and is connected to the signal output terminal 28. The free end of resistor 20 is connected to the free end of resistor 18 by the conductive element 30. The leg resistor 32 is deposited adjacent the innermost edge of resistors 18 and 20, but with varying width according to the relationship where W is the width of the deposited resistor 32 in meters, r is the radius of its outermost edge in meters, 1 is its thickness in meters, 0' is its clockwise rotation in radians, and p is its resistivity in ohm-meters. The relationship for W has been derived for an input and output impedance equal to fifty ohms and an attenuation range of zero to forty decibels for a clockwise rotation from Zero to two hundred seventy degrees.

A conductive strip 34 is deposited with uniform width on an arc having a lesser radius than the innermost portion of resistor 32. Conductive element 36 connects the narrowest end of resistor 32 to one end of the conductive strip 34 and the conductive element 30 which is common to resistors 18 and 20. Another conductive element 38 is connected to the widest end of the resistor 32 and forms a ground terminal for the attenuation network 10. Conductive arms 16 and 34, resistors 18 and 20' and the outermost edge of resistors 14 and 32 are all deposited concentrically by film evaporation techniques.

The electrical contacts 40 and 42 are fixed on the surface of a disc 44 which is similar in construction to the disc 12. Electrical contact 40 is positioned for continuous cooperation with the resistor 14 and the conductive strip 16 to make resistor 14 continuously variable. Similarly electrical contact 42 is positioned for continuous cooperation with the resistor 32 and the conductive strip 34 to make resistor 32 continuously variable.

The orientation of the electrical contacts 40 and 42, which is indicated, correspond to an attenuation setting of zero decibels relative to the orientation of the attenuation network 10. Clockwise rotation of the electrical contacts 40 and 42 through an angle of 270 place them in an orientation which corresponds to a maximum attenuation setting of forty decibels relative to the orientation of the attenuation network 10.

The discs 12 and 44 are concentrically mounted on a shaft 46 and disposed in a circular metallic housing 48 which serves as a ground plane for the strip line attenuation network 10. Disc 12 is fixed to the circular housing 48 so as to remain stationary with reference thereto and disc 44 is linked to the shaft 46 for rotation therewith. The electrical contacts 40 and 42 are maintained in contiguous relationship with the attenuation network 10 by means of a spring loading element 50 which is concentrically mounted on the shaft 46. This concentric spring loading of the electrical contacts 40 and 42 provides uniform pressure on the attenuation network 10 as the electrical contacts 40 and 42 are rotated. The uniformity of pressure is important as the electrical contacts 40 and 42 must make isotropic and homogeneous contact with the depositcd film attenuation network to guarantee a useful lifetime and maintain a low voltage standing wave ratio.

The predetermined geometric configurations of the resistors 14 and 32 are such that continuous rotation of the contacts 40 and 42 relative to the attenuation network 10 provides continuously variable attenuation from substantially zero decibels for a broad range of frequencies. For example, an attenuator constructed according to this invention can provide continuously variable attenuation from zero to forty db for a band of frequencies ranging from DC. to three kilomegacycles (gc.). Further, the configuration of the attenuation network provides a constant input and output impedance with variation in attenuation. In addition, the attenuator is low in cost and small in size because of its simple concentric construction.

Applicants invention is not limited to the exact details disclosed, but may be embodied in other forms. For example, the resistors 14 and 32 may be deposited with uniform width but continuously varying thickness. Further, similar relationships to those disclosed for the variation in width of resistors 14 and 32 may be derived to provide different ranges of attenuation and different angles of clockwise rotation.

I claim: 1. An attenuator having an input and an output and comprising:

an insulating substrate having deposited thereon a first path having varying resistance therealong and a second path having uniform resistance therealong,

means connecting the first and second paths between the input and output of said attenuator,

a third path having varying resistance therealon g deposited on said substrate,

means connecting one end of said third path to the second path at a point intermediate said input and out- P 7 means connecting the other end of said third path to a point of reference potential, and

means contacting said first and third paths and disposed for traversing the distance therealong to vary the attenuation provided by the attenuator. 2. An attenuator as in claim 1 wherein the resistance of the first and third paths varies therealong according to a relationship determined by the desired input and output impedance and the desired range of attenuation for a selected angle of rotation.

3. An attenuator as in claim 2 wherein the means contacting said first and third paths and disposed for traversing the distance therealong varies the electrical lengths thereof in opposite senses to provide substantially constant input and output impedance with variation in attenuation. 4. An attenuator having an input and an output and comprising:

an insulating substrate having deposited thereon a first path having varying resistance therealong and a second path having uniform resistance therealong,

means connecting the first and second paths between the input and output of said attenuator,

first shorting means having one end connected to the output of said attenuator and being deposited on said substrate adjacent to said first path,

a third path having varying resistance therealong deposited on said surface, means connecting one end of said third path to the second path at a point intermediate said input and output, means connecting the other end of said third path to a point of reference potential, second shorting means having one end connected to the distal end of said third path from said point of reference potential and being deposited on said substrate adjacent to said third path, means contacting said first shorting means and said first path to vary the electrical length thereof, and means contacting said second shorting means and said third path to vary the electrical length thereof in opposite sense from the variations in electrical length of said first path to provide substantially constant input and output impedance with variation in attenuation. 5. An attenuator as in claim 4 wherein said first, second, and third paths and said first and second shorting means are all concentrically deposited on said substrate which is enclosed within a ground plane. 6. An attenuator comprising: an insulating substrate having a bridged T attenuation network deposited thereon, the bridge and leg resistors of said network being concentrically deposited with uniform thickness and varying width according to a relationship determined by the desired input and output impedance and the desired range of attenuation for a selected angle of rotation, the'other resistors of said network being concentrically deposited with uniform thickness and width, said attenuation network including a conductive strip deposited adjacent each of said bridge and leg resistors, each of said bridge and leg resistors having one end connected in common with one end of its adjacent conductive strip, said attenuation network having an input and an output, a metallic housing which serves as a ground plane, said substrate being disposed within said housing, and electrical contacts rotatably disposed within said houssaid electrical contacts cooperating with said bridge and leg resistors and said conductive strips to provide continuously variable attenuation with low insertion loss in response to rotation of said contacts relative to said attenuation network.

References Cited UNITED STATES PATENTS 10/1959 Sommers et al 33384 5/1965 Weinschel 333-81 

1. AN ATTENUATOR HAVING AN INPUT AND AN OUTPUT AND COMPRISING: AN INSULATING SUBSTRATE HAVING DEPOSITED THEREON A FIRST PATH HAVING VARYING RESISTANCE THEREALONG AND A SECONE PATH HAVING UNIFORM RESISTANCE THEREALONG MEANS CONNECTING THE FIRST AND SECOND PATHS BETWEEN THE INPUT AND OUTPUT OF SAID ATTENUATOR, A THIRD PATH HAVING VARYING RESISTANCE THEREALONG DEPOSITED ON SAID SUBSTRATE, MEANS CONNECTING ONE END OF SAID THIRD PATH OF THE SECOND PATH AT A POINT INTERMEDIATE SAID INPUT AND OUTPUT, MEANS CONNECTING THE OTHER END OF SAID THIRD PATH TO A POINT OF REFERENCE POTENTIAL, AND MEANS CONTACTING SAID FIRST AND THIRD PATHS AND DISPOSED FOR TRAVERSING THE DISTANCE THEREALONG TO VARY THE ATTENUATION PROVIDED BY THE ATTENUATOR. 